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Old 11-07-2009, 07:39 AM   #1
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quick question about backflow preventer


Why do you need a backflow preventer in a hot water system when the city water isolation ball valve should always be closed unless filling or purging the system? From what I am learning the supply valve should always be off and if a low water condition ever occurred your 'low water cutoff' would indicate this as opposed to a slow leak of some sort being masked by the re-filling of city water.

So with the ball valve closed unless you are filling or purging... why the backflow preventer??

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Old 11-07-2009, 08:12 AM   #2
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quick question about backflow preventer


Small hot water boilers don't have low water cutoffs. Steam boilers do. Lots of hot water boilers have the valve open but the auto fill valves fail a lot(closed). There is a pressure relief valve on a hw boiler to protect you. If you lost lots of water you will notice a lack of heat. The plumbing code states that a backflow preventer is necessary no matter what. To protect any dirty water from the boiler under pressure going back to the city line if there is a water main break and loss of city pressure. The code is the code and MUST be followed.


Last edited by yuri; 11-07-2009 at 08:55 AM.
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Old 11-07-2009, 09:55 AM   #3
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quick question about backflow preventer


I had a 50k btu hot water boiler in which the inspector had to verify that I had a low water cut-off.
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Old 11-07-2009, 10:24 AM   #4
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quick question about backflow preventer


Depends on local codes. LWCutoffs are a good idea. A flow switch for the pump is even better.
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Old 11-07-2009, 01:19 PM   #5
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quick question about backflow preventer


The low water cuts off power to everything in the case of low water... what does the flow switch for the pump do? I've never heard of one of those. I'm still green, been trying to learn how all the components operate.

thanks -Ray
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Old 11-07-2009, 01:24 PM   #6
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quick question about backflow preventer


On Commercial size boilers they have them so the pump cannot run dry or cavitate.
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Old 11-07-2009, 05:12 PM   #7
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quick question about backflow preventer


When trying to purge or bleed a system. The autofedd is open, and unattended while your at teh rad, or baseboard.
So you need a backflow preventer.
Also, the autofeed could be left open by accident, or intentionally.

Yuri. Code across most of USA now requires LWCO on all boilers.
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Old 11-07-2009, 06:16 PM   #8
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quick question about backflow preventer


thanks guys I now realize that if its required to be there it is just reassurance that something bad, although unlikely, can happen.


I had another issue that I didn't want to start a whole new thread for. I was going through the process of sizing up components based on my system. So when it came to circulator pumps you need to figure out the longest loop, vertical or horizontal is irrelavent, but longest run to and from the boiler and add 50% for fittings. Then factor in which type of system you have i.e. baseboard, radiant... etc. So I did this and matched the appropriate circulator. I went to the plumbing supply and picked up my boiler (Utica dv-b 50K btu) and apparently it came with a circulator. Now I'm sure it will be fine for my system cause its only for a first floor two bed apartment (single loop) and boiler just below in the basement. But I thought it was odd that it comes with a circulator when there is a likely possibility that it won't suffice in many situations.
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Old 11-07-2009, 07:03 PM   #9
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quick question about backflow preventer


Yea. They all do that.
It will probably be a Taco 007, or B&G equivalent.
And often they are not the proper circ.
Manufacturers provide the circ, because most people can't figure out what circ to use.

A poor carry over practice from a long time ago. When most hydronic systems were gravity being changed over to force circulation.

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